Coal is a major fuel source with special significance to the United States which has 25% of the world's coal reserves Coal is formed from plant matter and can be as much as 70 weight percent water. Generally, older coal has less water than more recently formed coal. Coal is classified according to rank which correlates with its age and is based on its stage of coalification. High-rank hard coals such as bituminous and anthracite coals are formed in the final stage of coalification and have moisture content of 10 percent or less. See Karr, Analytical Methods for Coal and Coal Products, Academic Press, New York (1978), Vol. I at 247. Low-rank coals, such as brown coals and lignites, are more recently formed and in the intermediate stage of coalification. They typically have high moisture content ranging between 30 and 70 weight percent. Typically, coal from the western United States has as much as 50 weight percent water.
Moisture adds significant weight and can raise transportation costs accordingly. The per ton value of coal is determined by the net heat that can be obtained. The moisture content has the adverse effect of lowering the heating value of the coal because energy is lost in vaporizing the water. For these reasons, coal is dried before transportation to moisture contents typically below 10 percent. Freshly mined coal from the western United States, for example, can have a heating value of 8,400 BTU/pound compared to 10,860 BTU/pound after being thermally dried. However, the efficacy of the drying process is impaired by subsequent difficulties. The drying process causes shrinkage-induced stresses resulting in disintegration. This process is called decrepitation or slacking and can occur to a substantial degree within as few as 48 hours. See Berkowitz, An Introduction to Coal Technology, Academic Press, New York (1979) at 189. In addition to decrepitation, the coal is more susceptible to dust production when handled or transported. For some customers, the amount of dust produced by certain types of coal destroys the utility of those coals and therefore reduces the market for those coals. Furthermore, "windage" losses of mature coal from transport in open train cars is about 0.2 weight percent per 100 miles of travel. This windage loss has both economic and environmental costs.
Oil-based sprays have been used for years to coat coal and reduce fragmentation and dust production. The prevalent practice today is to apply #6 fuel oil or a mixture of #6 fuel oil and asphalt at rates greater than three gallons per ton of coal and typically 7 to 10 gallons per ton. These oil-based sprays have several drawbacks. For example, some have low flash points and can cause ignition during transportation or handling. In addition, they add weight to the coal when the primary purpose of drying the coal is to reduce unnecessary weight. Furthermore, their dust suppressant properties deteriorate with time. Typically, prior art oil-based dust suppressants have to be heated to reduce their viscosity in order to enable spray application. High viscosity is required to enable the oil-based suppressants to adhere to the coal, especially if the coal is wet. Dust suppressants also suffer reduced efficacy if they are absorbed below the surface of the coal and therefore incapable of suppressing dust generated from the outer surfaces.
Aqueous agents have been used, but they have not been found to produce satisfactory dust suppression in thermally dried coal and they are absorbed by the coal which reduces the heating value. In addition, aqueous suppressants can freeze in cold weather which makes handling difficult.
U.S. Pat. No. 2,005,512 issued to Vinz on Jun. 18, 1935 discloses a process of treating solid fuels to render them dustless. The process requires use of an asphaltic oil that is too viscous for room temperature application and requires heating to reduce the viscosity for spraying.
U.S. Pat. No. 2,319,942 issued to Miller on May 25, 1943 discloses dust-proofing coal spray compositions and methods that require heating and treatment with oxygen to affect viscosity. The compositions consist of oxygen-treated, non-paraffinic extract of uncracked petroleum lubricating oil stock. The disclosed method involves heating these compositions in order to lower viscosity for spraying solid carbonaceous lump fuel. Miller, therefore, entails added expense due to the special preparation that is required before application and the use of elevated temperatures during application.
U.S. Pat. No. 3,985,517 issued to Johnson on Oct. 12, 1976 discloses a process of crushing coal to particle sizes of less than one-half inch, heating the particulate coal with an inert gas that is heated to a temperature of 250.degree. to 500.degree. F., and simultaneously coating the coal with a heavy liquid hydrocarbon material such as a heavy hydrocarbon residual oil having a boiling range above 650.degree. F. The process requires the coal to be fed through a device which uses the warm gas to both heat and fluidize the coal in order to apply the heavy hydrocarbon dust suppressant.
U.S. Pat. 4,201,657 issued to Anderson on May 6, 1980 discloses a composition of aromatic hydrocarbon oil and asphalt that reduces dust loss and spontaneous combustion. It has a high viscosity to reduce runoff, a high flash point and a high initial boiling point. Anderson requires asphalt which while yielding some beneficial properties necessitates heating prior to application.
U.S. Pat. No. 2,333,543 issued to Gray on Aug. 28, 1945 discloses a coal dust suppressant containing lubricating oil solvent extract and blending oil. No disclosure is made of the use of petroleum resins.